A fuel cell has a structure in which an electrolyte membrane such as a polymer electrolyte membrane is interposed between an anode and a cathode. In the fuel cell, when fuel gas containing hydrogen contacts the anode and oxidizing gas containing oxygen such as air contacts the cathode, an electrochemical reaction occurs at each of both electrodes, whereby an electromotive force is generated. In a fuel cell system in which electric power is obtained by such a fuel cell, it is important to minimize unnecessary consumption of hydrogen used as fuel, in terms of energy saving. Accordingly, in a conventional fuel cell system, off-gas of the fuel gas used in the fuel cell is re-circulated in the fuel cell, whereby efficient use of the hydrogen remaining in the off-gas is promoted.
In the fuel cell, water is generated by an electrochemical reaction at the cathode, that is, a reaction of hydrogen ions which have permeated through the electrolyte membrane from the anode side and oxygen contained in the oxidizing gas. Although a major portion of the water generated by the reaction is contained in the off-gas of the oxidizing gas and then released into the atmosphere, part of the generated water permeates through the electrolyte membrane and moves to the anode side. Although the generated water moved to the anode side is contained in the off-gas of the fuel gas and then discharged from the fuel cell, the off-gas is re-circulated in the fuel cell as fuel gas. Accordingly, water is accumulated in a fuel gas circulation system in which fuel gas is circulated. Accumulation of water in the circulation system inhibits supply of hydrogen to the anode, causing deterioration of power generation performance of the fuel cell.
In order to address this problem, Japanese Patent Application Publication No. JP-A-2002-313403 discloses a technology in which a drain device (a gas/liquid separation device) is provided in a fuel gas passage, and water contained in the fuel gas is collected by the drain device. A valve (discharge valve) is provided in a lower portion of an accumulation portion of the drain device. The water accumulated in the accumulation portion can be discharged to the outside of a fuel cell system by opening the valve. In this technology, a water level of the water accumulated in the accumulation portion is measured by a water level sensor, and the valve is opened/closed based on the measurement value.
As disclosed in each of Japanese Patent Application Publication No. JP-A-2002-216812 and Japanese Patent Application Publication No. JP-A-2002-289237, a system is known in which a purge valve is provided in a fuel gas circulation system, and nitrogen accumulated in the circulation system can be discharged to the outside of the fuel cell system along with fuel gas. Nitrogen is a main component of the oxidizing gas supplied to the cathode. When part of the oxidizing gas permeates through an electrolyte membrane from the cathode side to the anode side, nitrogen is accumulated in the circulation system. An increase in a nitrogen concentration in the fuel gas decreases a hydrogen concentration, and therefore reduces power generation performance of a fuel cell. Accordingly, in the system disclosed in Japanese Patent Application Publication No. JP-A-2002-216812 and the like, the nitrogen is discharged to the outside of the fuel cell system along with the fuel gas by opening the purge valve when a predetermined condition is satisfied.
In a system disclosed in Japanese Patent Application Publication No. JP-A-2002-313403, in terms of maintenance of power generation performance of a fuel cell, it is important to make it possible to discharge nitrogen from a fuel gas circulation system. Therefore, it is preferable to provide a purge valve in the system disclosed in Japanese Patent Application Publication No. JP-A-2002-313403 as well. In this case, it is also desired that the structure of the system be simplified by endowing the valve of the drain device with the function of the purge valve, namely, both the accumulated water and the fuel gas be discharged by using the valve of the drain device.
However, the system disclosed in Japanese Patent Application Publication No. JP-A-2002-313403 has a problem in using the valve of the drain device also as the purge valve. In the system disclosed in Japanese Patent Application Publication No. JP-A-2002-313403, an amount of discharged water can be measured by using the water level sensor. However, when the fuel gas is also discharged, an amount of discharged fuel gas cannot be measured. If the amount of discharged fuel gas cannot be obtained, the hydrogen concentration in the fuel gas may not be recovered sufficiently, or hydrogen may be discharged unnecessarily.
The system disclosed in Japanese Patent Application Publication No. JP-A-2002-313403 has a problem also in discharge of the accumulated water. When the system disclosed in Japanese Patent Application Publication No. JP-A-2002-313403 is an in-vehicle system for an electric vehicle or the like, a fluid level of the water in the accumulation portion changes due to a tilt, vibration and the like of the vehicle. Therefore, a water level cannot be accurately measured by the water level sensor. If a water level cannot be measured accurately, it is difficult to reliably discharge the accumulated water.